The release notes contain the significant changes in each
PostgreSQL release, with major
features and migration issues listed at the top. The release
notes do not contain changes that affect only a few users or
changes that are internal and therefore not user-visible. For
example, the optimizer is improved in almost every release, but
the improvements are usually observed by users as simply faster
queries.

A complete list of changes for each release can be obtained by
viewing the Git logs for each release. The
pgsql-committers email
list records all source code changes as well. There is also a
web interface that shows changes to specific
files.

The name appearing next to each item represents the major
developer for that item. Of course all changes involve community
discussion and patch review, so each item is truly a community
effort.